1. Field of the Invention
The invention relates to a process for conditioning moist and dust-laden incineration air in incineration plants and, in particular, waste incineration plants, in which the incineration air is sucked in from a dust-containing and moist environment and is conveyed through an airpreheating device to the furnace. The invention also relates to an apparatus for carrying out the process.
2. Description of the Related Art
In the operation of incineration systems, in which the incineration air is sucked in from a dust-containing and moist environment, for example from the bunker for the waste substances in the case of waste incineration systems, there is the problem of the heat exchanger surfaces of the preheating devices used for the incineration air becoming soiled. Preheating devices of this type for the incineration air are used in order to preheat the incineration air for the furnace and therefore to improve the incineration process or to increase the efficiency. Heat is supplied by hot water or steam, which is generally produced in incineration plants of this type. Increasing soiling of preheating devices of this type for the incineration air causes a deterioration in the heat transfer between the heating surfaces of these devices and the incineration air to be heated and an increase in the flow resistance through these preheating devices for the incineration air, which leads to a rise in the driving power for the fan of the incineration air. In order to eliminate soiling of this type, cleaning devices, such as compressed-air or steam blowers, rotating cleaning brushes and bypass ducts to the preheating devices for the incineration air are provided. In this case, the bypass flow to the preheating device for the incineration air runs parallel to and in the same direction as the air flow through this air-preheating device. The deterioration of the incineration process or in the efficiency of the preheating device for the incineration air in bypass mode has to be accepted. The soiling is therefore not prevented or removed, but the necessary cleaning process is deferred just temporarily. This cleaning process has to be carried out at the latest when the power reserve of the fan has been exhausted.
This soiling of the preheating device for the incineration air, which is extremely annoying and is associated with the abovementioned disadvantages, is arrived at because the moist dust particles remain stuck to the hot heat exchanger surfaces. This moisture on the surface of the dust particles has various causes. One cause is that in spite of a dry surface the dust particles are still moist in the interior and during impact against the heat exchanger surfaces this moisture passes to the surface and moistens it again, which leads to the abovementioned sticking-on effect. A further cause can be found in that as the water on the surface of the dust particles evaporates the incineration air is cooled, which leads to the air which is to be preheated falling below a local dew point. However, when it falls below a dew point, moisture precipitates out of the air and this moisture then leads to the dust particles being moistened again, with the result that baking-on of these dust particles on the heat exchanger surfaces is unavoidable.
The object of the invention is to provide a process and an apparatus, with the aid of which the soiling of the preheating device for the incineration air can be largely avoided.
According to the invention, this object is achieved in a process of the type mentioned at the beginning by heated air being mixed with the sucked-in incineration air before its entry into the air-preheating device. This measure has at any rate the consequence of sufficiently drying the moist dust particles so that they no longer, or only to a very small extent, remain stuck to the heat exchanger surfaces of the air-preheating device. By this means, the problems explained at the beginning are largely or completely eliminated and therefore the overall efficiency of the incineration plant is ultimately also ensured.
In order to be able to implement the proposed process in a reproducible manner for large-scale incineration plants, it is advantageous, in a development of the invention, if at least one of the parameters comprising temperature, relative air humidity and pressure of the sucked-in incineration air is measured before and/or after the mixing in and is used as a regulating variable for determining at least one of the parameters comprising amount, temperature, relative air humidity and pressure of the heated mixing air. The temperature and the relative air humidity of the sucked-in incineration air is preferably measured before and after the heated air is mixed in and the necessary measures for the regulating process are derived therefrom. In particular, the amount of heated air to be mixed in is determined by this means.
In an advantageous manner, air from the air-preheating device is used as mixing air. In this connection, the air heated in the air-preheating device is branched off via a bypass and mixed with the sucked-in incineration air before its entry into the air-preheating device. In addition to obtaining the desired drying of the dust particles, this measure also has the further advantage that the dry dust particles contained in the heated incineration air become attached to those dust particles which have not yet dried completely, with the result that these dust particles which have not yet dried are surrounded by a layer of dried dust particles, which leads to a reduction in the tendency for them to bake onto the hot heat exchanger surfaces.
If the sucked-in incineration air is laden with dust to a particularly severe extent, it may be advantageous, after heated mixing air is fed to the sucked-in incineration air and before this mixed incineration air is introduced into the air-preheating device, to undertake dust removal. Dust particles which have already dried can be removed more unproblematically, for example through a filter, than moist dust particles.
An apparatus for carrying out the process is characterized in that within a mixing chamber there are arranged feed pipes for the mixing air, which pipes run transversely with respect to the direction of flow of the incineration air, are closed at the ends and have outlet openings for the mixing air. The heated mixing air emerges from the openings of the feed pipes and becomes uniformly mixed with the sucked-in incineration air flowing through the mixing chamber.
A preferred embodiment of the apparatus for mixing heated mixing air with the sucked-in incineration air is characterized in that each feed pipe is provided with three rows of air slots which are mutually offset by 120xc2x0 in each case on the circumference, one row of air slots being provided on that side of the feed pipe which is placed in the main direction of flow of the incineration air, in that a casing which tapers conically in the main direction of flow of the incineration air is arranged on the feed pipe and forms a drop-shaped flow body together with the feed pipe, and in that a row of air slots are formed at the tapered end of the casing. In the abovementioned arrangement of rows of air slots, two rows of air slots are directed at an angle of 60xc2x0 in each case counter to the main direction of flow of the sucked-in incineration air, with the result that mixing of the emerging, heated mixing air with the sucked-in incineration air takes place virtually by the counter-flow principle. The oblique setting of the blow-out direction of the two upper rows of air slots causes the sucked-in incineration air to be placed around the feed pipe and to be conducted away in a manner favourable in terms of flow through the arranged, conically tapering casing. By means of the arrangement of the air slots facing away in the direction of flow, the conically tapering casing is heated and bounds a chamber in which the dust particles entrained by the heated mixing air can precipitate. This fraction of the mixing air then emerges through the air slots arranged at the end of the tapered casing and then also carries along the removed dust particles. The casing surfaces form a heat exchanger surface for heating the sucked-in incineration air.
A preferred refinement of the apparatus is characterized in that the feed pipes are arranged within the mixing casing in a horizontal direction with the casings orientated downwards, and in that the direction of flow of the incineration air within the mixing chamber runs from the top downwards.
The invention will be explained in greater detail below with reference to an exemplary embodiment illustrated in the drawing, in which: